Refactor music item ontology

Ideally, our classes referencing music items (e.g. TopMusic, Track) should be refactored such that we have a single 'MusicDetailsFragment', since they have similar use-cases. I started work on this awhile back (by creating an abstract MusicEntity class) but ran into some issues.

Namely, when data-binding the MusicDetails layout, we would need to know at runtime whether we're binding TopMusic or Tracks. I'm sure there's a canonical way to do this, but binding through the XML seems problematic with this approach.

Start by locating TopMusic, Track, the existing MusicEntity abstraction, MusicDetailsFragment, and the MusicDetails data-binding layout. Read how those classes are currently used and determine whether one fragment can support both item types without problematic XML binding. Done means the shared design is implemented and both music-item use cases bind correctly.

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